Best way to build an online booking platform

Hi everybody!

A little history about me first...

I used to be a web designer and then I moved industries after many successful years of building small websites for local businesses.

I have decided to get back into the web design game and I'm currently refreshing all of my html, css & php knowledge using Treehouse.

I have a business idea I'd like to launch and it will require me to build and manage a booking system for clients. The booking system needs to sell memberships and allow customers to book onto fitness classes. There are systems like this on the market I know but I'd really like to build it from the ground up if possible.

My coding experience is basic, as I previously mentioned I'm refreshing html, css & php and whilst I'm fairly confident with these languages it would be great to hear from you guys which other languages I should start to master to achieve my goals.

Thanks for your time, and I look forward to hearing from you guys.

Kind regards,

Ash